渭河中游宝鸡—扶风北岸斜坡结构及其对大型滑坡形成机理的指示意义

Slope Structure on the North Bank of Baoji-Fufeng Section in the Middle Reaches of Weihe River and Its Indicative Significance on the Formation Mechanism of Large Landslide

渭河中游宝鸡—扶风段北岸塬边集中发育大型黄土滑坡,体积超过1 000×104 m3的滑坡达58处。在区域活动断裂调查、地球物理勘察、典型斜坡结构与构造调查基础上,研究了大型滑坡结构及其形成机理。研究表明:渭河北缘活动断裂以地堑式结构控制北坡塬边斜坡的地貌,主断裂面倾向南,倾角平均为68°,而次级断面结构组合控制斜坡结构,影响塬边大型黄土滑坡结构;塬边主断裂面与多个大型黄土滑坡后缘滑壁一致,断裂最大下挫距离7.1m,影响大型滑坡的形态及发生过程;活动断裂呈地堑式结构,局部次级断面控制塬边多级滑动面的结构及其演化过程,如杨家村滑坡,局部滑坡剪出受断裂结构控制,剪出口角度可达72°,大部分剪出口未受到断裂面影响,角度平均为23°;三门组黏土岩黏粒体积分数超过35%,塑限平均值为23,是滑坡的层间剪切带;降雨诱发老滑坡复活是目前新滑坡发育的主要形式,大量的老滑坡形成于0.009Ma BP二级阶地侵蚀期,而古滑坡形成于0.13Ma BP三级阶地侵蚀期。

The plateau edge on the north bank of Baoji - Fufeng section in the middle reach of Weihe River is the concentrated development zone with large loess landslide, which consists of 58 places each with the area above 10 million cubic meters. Based on the detailed survey on the regional active fractures, geophysical survey, and detailed survey on typical slide structures, the authors studied the slope structure and mechanisms of landslides along the plateau edge. The study shows that： 1）Active faults in northern margin of the Weihe River, as graben-like structure, control the terrain and physiognomy of slope zone of the plateau edge on the northern bank. The details are that principal fracture surface is S-dipping and the dipping angle is 68~ on average controls the angle of plateau edge in bank slope; and the assemblage of secondary faulted structure controls the slope structure and affects the structure of large loess landslide on the plateau edge. 2）The principal fracture surface of the plateau edge is consistent with smooth wall of back scarp of many large loess landslide, and it has influence on the configuration and occurrence of large landslide. The maximum distance o{ fault offset is 7.1 m. 3） Active faults have graben-like structure and local secondary fracture surface controls the structure of multi-order sliding surface of plateau edge and its formation and evolution. For instance, in the Yangjiacun landslide, local landslide is controlled and sheared by faulted structure and the shearing angle reaches 72~. The toe of surface of rupture of most landslides are not affected by fracture surface and their angles are 23~ on average. 4）The clay content of clayey layer in the Sanmen Formation overpasses 35% and the average plastic index is 23. It is an interlayer shear zone of landslide. 5）The major pattern of new landslide development is reactivation of old ones induced by the precipitation. Many old landslides on northern bank were formed in the denudation period of second-order terrace in 0.009 Ma BP, while ancient landslides in the period of third-order terrace in 0.13 Ma BP.